1. Technical Field
Fuel cells have been championed as viable alternatives over existing battery technology for portable electronic devices; however, a key issue for widespread adoption and success of fuel cell technology involves addressing the meager performance of these devices due to poor efficiency and durability of the catalysts. The present disclosure is generally directed to bulk metallic glass materials for energy conversion and storage applications. More particularly, the present disclosure is directed to a new class of materials that can circumvent Pt-based anode poisoning and the agglomeration/dissolution of supported catalysts during long-term fuel cell operation. An exemplary implementation of the present disclosure involves use of Pt58Cu15Ni5P22 bulk metallic glass to create a new class of high performance nanowire catalysts for use in fuel cell applications.
2. Background Art
Amorphous alloys were developed approximately fifty years ago following reports concerning the formation of Au-Si metallic glass [1]. Researchers developed rapid quenching techniques for chilling metallic liquids at cooling rates of 105-106° K/s. However, these high cooling rates limited the potential geometries of these alloys to thin sheets/lines and stymied the range of potential applications [2]. Recently, the development of several multi-component alloys capable of solidifying into glass at relatively low cooling rates (1˜102° K/s)—materials that vitrify without crystallization—has permitted the production of large-scale bulk metallic glass (BMG) samples on the order of 30 mm [3]. These BMGs represent a new class of engineering materials with an unusual combination of strength, elasticity, hardness, corrosion resistance and processability [4-8]. The random atomic structure in BMGs is devoid of dislocations and associated slip planes. These systems can result in elasticities of 2% (Zr-based BMG formers [6]) and yield strengths of up to 5 GPa (Co-based BMG formers [9]). During yielding, however, BMGs can suffer from macroscopically brittle failure at ambient temperatures [10]; however, when samples with small dimensions are characterized, significant global plasticity is observed [11-13].
A wide range of BMG-forming alloys have been developed, including Zr-[14-16], Fe-[17, 18], Cu-[19], Ni-[20], Ti-[21], Mg-[22], Pd-[23], Au-[24] and Pt-based compositions [25]. As discussed by Wang et al. [2] and Schroers [26], applications for these materials can range from thermoelectric devices to biocompatible implants and have already impacted fields ranging from sports (i.e., tennis rackets, golf clubs etc.) to Micro/Nano Electromechanical systems (MEMs/NEMs) devices.
One of the challenges with BMGs is that the current approach towards developing these materials to exhibit specific properties is carried out by synthesizing and characterizing each alloy composition individually. The common strategy is a trial and error approach that can result in hundreds of time-consuming experiments [24, 25]. Such an approach is highly inefficient towards rapid identification of bulk metallic glass forming compositions, limiting advancements in this field. A combinatorial approach could provide an elegant solution to the task of mapping systems that could form new bulk metallic glass alloys with desirable properties. Successful combinatorial techniques have previously been developed in the pharmaceutical industry [27] and are now being considered as a viable approach for mapping alloys across compositional phase diagrams [28, 29]. For example, Sakurai et al. used a combinatorial arc plasma deposition (CAPD) to search for Ru-based thin film metallic glass by making libraries. Each library consisted of 1089 CAPD samples deposited on a substrate [30].
With specific reference to fuel cell technology, a fuel cell electrode has three primary functions: (i) allowing access to reacting gases, (ii) providing active electrocatalytic sites, and (iii) allowing transport of electrons as well as ions. Electrical power is generated by oxidizing the fuel electrochemically, e.g., by digesting carbon-based fuels with the help of an internal catalyst. However, poisoning and/or deposition on the anode can significantly interfere with the operation/efficiency of fuel cell systems. For example, sulfur is a potent poison for nickel electrocatalysts present in many current anodes. Similarly, conventional anode technology—which generally involves anodes fabricated from porous carbon coated with platinum—is highly susceptible to impurities in the hydrogen fuel which, if present, easily bind with the platinum, “poisoning” the electrode and decreasing fuel cell performance Carbon deposition, which reduces the activity of the anode, can occur if the steam-to-carbon ratio of the fuel gas is too low. Nickel effectively functions as a catalyst for carbon deposition (coking), thereby blocking the active sites of the anode and, in the worst case, destroying its structure. Technologies/techniques are needed that ensure durability and efficiency of fuel cell operation, despite the potential for poisoning and/or deposition phenomena interfering with anode functionality.
The role of surface chemistry in catalytic development is significant. The surface chemistry of a material in the praxis of a metal/electrolyte interface can be described by either heterogeneous or electrochemical reactions where one can term the activation surface a catalyst. The activity of the catalyst can be due to structural or chemical modifications of the electrode surface and additions to the electrolyte. Structural effects can be caused by variations in the electronic state and by variation in the geometric nature (i.e. crystal planes, clusters, alloys, surface defects) [46]. To correlate the electrocatalytic ability with a physiochemical property of a material, plots can be made of electrochemical activity (either current density at constant potential, or potential at constant current density) versus the physiochemical property. Balandin first proposed these as volcano plots [47], if the resulting plot is a bell curve (see FIG. 4—adapted from [48]).
Heterogeneous catalysis is relevant to the design and operation of direct alcohol fuel cells. The optimum heterogeneous catalyst will provide the correct reaction site geometry, along with the proper electronic environment, to facilitate the reaction of interest. An example of this can be found from catalyst development in direct alcohol fuel cells where a major challenge is the search for efficient electrocatalysts that would remedy the Pt-based anode poisoning by a carbonaceous intermediate (most likely CO) during alcohol oxidation. Direct alcohol fuel cells are of particular interest because of the high power density of liquid fuels (e.g. methanol, ethanol). The best catalysts for methanol oxidation are based on Pt—Ru systems. However, the high cost of Ru has led to research aiming to identify other less expensive metals, M, that exhibit enhancement of Pt or Pt—Ru catalytic activity. A guide to the design of efficient Pt-M methanol oxidation binary systems is provided in a review by Ishikawa et al., where the theoretical predictions for the effect of the second metal is provided by three key reaction steps (methanol dissociative chemisorption, CO poison adsorption, and CO removal via its oxidation by adsorbed OH) [49]. The effects of these Pt-M systems can be grouped into two main categories:                1. Ligand effect—modification of the Pt electronic properties by the second metal [50].        2. Synergistic effect—bi-functional mechanism whereby the second metal disrupts the continuity of the Pt lattice and provides sites for OH adsorption [51-53].        
More recently, the electronic effect has been studied using the density functional theory to estimate the direction and extent of the d-band energy center, ed, shifts when metals of different Wigner-Seitz radii and electronegativity are found together [54-56]. Of note, the addition of Cu, Fe, Co, and Ni to Pt results in a Pt ed down-shift, which in turn is both theoretically predicted and experimentally found to lead to decreased CO adsorption [57-61].
Corrosion challenges are substantial with conventional fuel cell catalysts. The surface chemistry activity is also related to the structural stability and corrosion of the surface. For instance, in direct alcohol fuel cells, electrocatalyst durability has been recently recognized as one of the most important issues that must be addressed prior to direct alcohol fuel cell commercialization [62, 63]. Presently, the most widely used catalyst system is platinum in the form of small nanoparticles (2-5 nm) supported on amorphous carbon-particle aggregates (Pt/C). The poor durability of the Pt/C catalyst is evident by a fast and significant loss of platinum electrochemical surface area (ECSA) during the time of fuel cell operation [62-64]. The mechanisms for the loss of platinum ECSA can be summarized as follows[62, 63]:                1. Loss of Pt nanoparticles from the electrical contact due to corrosion of the carbon support.        2. Pt dissolution and redeposition (migration of the soluble Pt+ species within the polymer electrolyte and the eventual chemical reduction by hydrogen crossover from the anode through the proton exchange membrane.        3. Ostwald ripening (Pt nanoparticle aggregation driven by surface energy minimization.)        
For these reasons, there has been considerable recent interest in the development of nanowire fuel cell catalysts [65-67]. Previous nanometallic synthesis efforts have focused on bottom up assembly through the reduction of salt precursors or electrochemical deposition processes to create the following: Pt and Pd nanotubes [68], Au-Ag nanoporous nanotubes [69], NiCu [70], PtCo nanowires [71], Pt3Ni(111) single crystals [72], and Pd-Pt bimetallic nanodendrites [73]. Many of these strategies involve complex synthesis methods due to the difficulty in forming metallic alloys into the nanometer-length scales necessary for maintaining a high dispersion (noble metal utilization). Of further note, Chen et al. reported an enhanced durability for pure platinum nanotubes (PtNTs) and suggested that the activity of the PtNTs could be further improved by employing platinum alloy nanotubes [68].
Based upon these initial challenges, there is a clear need for a new type of catalyst material that does not suffer from durability issues and that displays the high electrochemical activity consistent with a multi-component catalyst system. Bulk metallic glasses (BMGs) are of particular interest for these kinds of surface chemistry studies because the surface and structure of these alloys can be patterned down to the same scale as conventional supported catalysts [26]. The absence of crystallites, grain boundaries, and dislocations in the amorphous structure of bulk metallic glass results in a homogeneous and isotropic material down to the atomic scale, which displays very high strength, hardness, elastic strain limit and corrosion resistance. BMGs represent a positive step in this direction as these amorphous metals can be formed into nanowires (FIG. 3d) that circumvent the very complex synthesis, low throughput, low reproducibility, and high cost typically associated with nanowire fabrication [74-77].
These and other objectives are satisfied according to the systems and methods of the present disclosure.